The invention relates to a method for supplying persons extremely hard of hearing with acoustical signals according to the generic (introductory) part of claim 1 and devices for implementing this method. Such methods and devices are known, for instance, from the U.S. Pat. No. 3,385,937.
A hearing aid with a microphone for conversion of the acoustical signals received into electrical signals is known from the aforementioned reference, in which those signals which are allowed to pass by filters are employed for the modulation of an electric auxiliary AC voltage, the resultant modulated a.c. signal being supplied after amplification and conversion as acoustical signals in an ear piece for the ear to be supplied. In such a system, the filters are to be designed such that they only allow signals to pass whose frequencies either lie between 1500 and approximately 3500 Hz or between a first value of the range 4500 through 6000 Hz and a second value of the range 7000 through 8000 and that the frequency of the electrical carrier voltage lies between 350 and 1000 Hz. The part of the signals coming from the microphone lying below approximately 1000 Hz can be added to such a modulated signal or, respectively, to a pair of such modulated signals. Such hearing aids, however, have not been able to prevail in hearing device technology since, for a single filter, the filter breadth 1500 Hz through 3500 Hz is too broad and, upon employment of two filters, the filter breadths are too small and important speech information is not placed at the disposal of the hard of hearing.